1. Field of the Invention
This invention relates to a fluid polishing method and a fluid polishing apparatus for executing the fluid polishing method. More particularly, the present invention relates to a method, and an apparatus for the method, for highly precisely processing a fine aperture by using a slurry of a polishing material.
2. Description of the Related Art
A large number of apparatuses exist that have a high precision fine aperture such as a nozzle tip of a fuel injector, a jet port of a carburetor, an orifice for regulating a fluid flow rate, a jet nozzle of a printer, and so forth. Methods for processing such a fine aperture include methods which employ laser, electron beam and discharge processing. There is the case where fluid polishing is employed when sufficient precision cannot be achieved by such methods. An example of the use of the fluid polishing method is a processing of a fine aperture of an orifice of a fuel injector for a diesel engine common rail. A common rail construction has recently been employed for diesel engines, and the diesel engines have been mounted to a variety of automobiles ranging from compact cars having an output of about 80 kW to large-scale trucks. However, fuel efficiency drops, and this adversely affects the economy, if any flow rate error occurs in the fuel injector. At the same time, pollutants of the environment increase undesirably in the exhaust gas and this is not desirable.
The flow rate error of the diesel engine common rail injector is greatly affected by the accuracy of the static oil flow rate of the orifice, as one of its constituent components, and a metering processing has been carried out by fluid polishing. Fluid polishing is carried out by causing a slurry (mixture of abrasives and an oil), discharged from a cylinder by the movement of a piston, to flow through the orifice to enlarge the diameter and to form an inlet R. However, there are cases where the oil flow rate greatly exceeds a target flow rate to thereby invite a defect, or is greatly smaller than the target flow rate and requires the repetition of fine adjustment.
The prior art technology has proposed a polishing method of a fine aperture of a nozzle of a diesel fuel injector (for example, Japanese Translation of PCT application 11-510437). A fluid polishing apparatus used for such a fluid polishing method includes a slurry tank and a cylinder for feeding the slurry.
The possibility of the occurrence of switching the cylinders, during the processing, exists in the fluid polishing method according to the prior art. In the fluid polishing apparatus, a piston of each cylinder moves back to suck the slurry from the tank after the slurry is used up. Because the suction time exists, the equipment has two cylinders so that as soon as the slurry of one of the cylinders is used up, the other cylinder is used. Nonetheless, the processing pressure cannot be kept constant at the instant of switching and a pressure fluctuation develops. If the pressure rises instantaneously, the flow rate apparently rises with reference to the relation Q=A·√P (A: constant, P: pressure) and reaches a target flow rate. Consequently, suitable processing is not carried out and the actual oil flow rate becomes smaller (see FIGS. 3 and 4). When the timing of switching of the cylinders occurs at the time that the flow rate is in the proximity of the target slurry flow rate (smaller by about 1 to 2 cc/min), the oil flow rate becomes large, on the contrary. As the processing capacity of fluid polishing is proportional to the pressure, the processing is promoted as the pressure becomes higher at the instant of switching and the processing that should originally be finished after a little time is excessively executed. As a result, the oil flow rate becomes larger (see FIG. 4). To solve this problem, it may be conceivable to drastically increase the cylinder capacity and to reduce the frequency of switching. When the cylinder capacity is increased, however, the slurry may be separated and the abrasives may precipitate inside the cylinder, so that variance occurs in the processing capacity and the precipitated abrasives are solidified and clog the cylinders.